1. Technical Field
This invention relates generally to spark plugs and more particularly to contact glass compositions for use in resistor-type spark plugs.
2. Discussion
All internal combustion gasoline engines employ spark plugs to generate the spark that ignites the air-fuel mixture in the cylinder. Spark plugs are comprised of three basic components: the shell, the insulator, and the electrodes. A resistor-type spark plug has an additional component: the resistor.
With reference to the FIGURE, there is illustrated a cross-sectional view of a conventional resistor-type spark plug 10. A shell 12 having an upper hexagonal-shaped section 14 and a lower threaded section 16 is typically comprised of a metallic material such as steel. The hexagonal-shaped section 14 is used to apply installation torque, while the threaded section 16 allows the spark plug 10 to be conveniently screwed into the cylinder head.
The shell 12 surrounds an insulator 18 which is typically comprised of a refractory or ceramic material, such as aluminum oxide. Insulators must be able to resist heat, cold, chemical corrosion, vibration, and high voltage changes.
A center bore 20 extends longitudinally through the central axis of the insulator 18. A terminal rod or stud 22 is disposed in the upper portion of the center bore 20. The top portion of the terminal stud 22 serves as an attachment point for the spark plug wire assembly.
A center electrode 24 is disposed in the lower portion of the center bore 20. The center electrode 24, as well as other components of the spark plug 10, carries the high-voltage current from the ignition coil and is insulated from the rest of the spark plug 10 by the insulator 18. A side or ground electrode 26 is attached to the shell 12 and is bent inwardly to produce the proper spark gap G between the two electrodes. Once a sufficient amount of voltage has built up, a spark is initiated in the electrode gap G and results in the ignition of the air-fuel mixture in the cylinder. Because the typical modern spark plug is expected to last from 100,000-150,000 miles or more, the electrodes must be constructed of materials that will be resistant to heat, oxidation, erosion, and corrosion. Typical materials used to make spark plug electrodes include alloys of metals such as iron, chrome, nickel, and platinum.
The spark at the electrodes is delivered in two stages. The voltage at the center electrode 24 will rise rapidly until the voltage is sufficient to ionize the gap G and cause the spark plug to fire. This is known as the first stage and is generally capacitive in nature. The second stage is longer and immediately follows the first stage. The second stage is produced by the remaining residual voltage in the ignition coil and is generally inductive in nature.
The combustion process takes place during the first stage. The second stage causes undesirable electromagnetic interference with radio and television communication equipment and other electronic devices. In order to shorten the second stage, it has become increasingly common for spark plug manufacturers to employ a suppressor or resistor of around 5,000-10,000 ohms. The resistor 28 is typically disposed in the center bore 20 between the terminal rod 22 and the center electrode 24 and is surrounded by the insulator 18. Additionally, two zones of electrically conductive contact glasses 30 and 32 are typically located on either side of the resistor 28, thus defining an upper interface 34 and a lower interface 36 with the resistor 28. The resistor 28 and the two zones of electrically conductive contact glasses 30 and 32 are generally referred to as the resistor body. On one end of the resistor 28 is the electrically conductive contact glass 32 in contact with the center electrode 24, and on the other end of the resistor 28 is the other electrically conductive contact glass 30 in contact with the terminal stud 22. The resistor 28 is positioned inside the insulator 18 either through a filling, tamping and pressure sealing process, or by a pressure sealing process using preformed resistor cartridges.
The chemical composition of resistors may vary widely. For example, some resistors are comprised primarily of a mixture of carbon-based materials and one or more types of glasses, with the resulting mixture being referred to as a resistor glass. These carbon-based resistors are generally referred to as carbon resistors. A carbon resistor designated ES-533S is employed in certain spark plugs marketed under the registered trademark AUTOLITE(copyright) by AlliedSignal, Inc. (Morristown, N.J.). These carbon resistors are comprised primarily of a mixture of thermal carbon, lamp black carbon, zirconia (typically in powder form), mullite (typically in powder form), and borosilicate glass (typically in powder form).
Like the resistor, the chemical composition of contact glasses may also vary widely. For example, a contact glass composition designated ES-534 is employed in certain spark plugs marketed under the registered trademark AUTOLITE(copyright) by AlliedSignal, Inc. (Morristown, N.J.) having a composition of about 40-45 weight % nickel (typically in flake form), about 45-50 weight % borosilicate glass (typically in powder form), about 10 weight % mullite (typically in powder form), as well as very small amounts of binder materials (e.g., about 0.5 weight % of VEEGUM(copyright) Tee (xe2x80x9cVGTxe2x80x9d)) and other metallic materials (e.g., about 0.3 weight % of copper (typically in flake form)).
In the pressure sealing process, the resistor glass mix and the contact glass mix are compressed at a temperature around 1800xc2x0 F. After sealing, the nickel flake is compacted into a dense network distributed around the compressed borosilicate glass powder particles, and provides paths for the flow of electric current between the center electrode/terminal stud and the resistor glass. The degrees of compression of the nickel flakes, the number and nature of contact points of the conductive elements at and near the interface, and the chemistry and microstructure of the interface can all significantly affect the durability and stability of the resistor.
In vehicle applications, certain resistor-type spark plugs, especially those employing carbon resistors, were found to become non-functional due to degradation of the resistor bodies. Some were found to have their resistance values increase from a nominal value of 5000 ohms up to 1,000,000 or more ohms. In essence, the resistor, or portions thereof, began to function as an insulator. These resistance increases were the result of substantial melting of the resistor glass. The degradation started with local arcing at the interface between the resistor glass and the contact glass closest to the terminal stud. The local arcing eventually led to a large volume melting of resistor glass next to the interface with the contact glass. Other spark plugs were observed to exhibit irregular voltage discharge across the electrode gap. The degradation also started at the contact glass/resistor glass interface with local arcing. The local arcing subsequently led to a number of open channels, providing paths for internal arcing inside the resistor glass.
It is believed that the composition of the contact glass can have a profound effect on the durability and stability of the resistor. The contact glass composition can alter the physical and chemical properties of the interface between the contact glass and the resistor glass. This is particularly true at the entrance closest to the terminal stud where more material flow has taken place during the pressure sealing process.
Therefore, there exists a need for an electrically conductive contact glass composition that provides durability and stability to spark plug resistors.
In accordance with one embodiment of the present invention, a contact glass composition for use in resistor-type spark plugs, comprises a mixture of (a) at least one electrically conductive material selected from the group consisting of nickel, copper, iron, zinc, titanium, silver, and oxides thereof; (b) graphite; and (c) silicon.
In accordance with another embodiment of the present invention, a contact glass composition for use in resistor-type spark plugs, comprises a mixture of (a) at least one electrically conductive material selected from the group consisting of nickel, copper, iron, zinc, titanium, silver, and oxides thereof; (b) graphite; (c) silicon; (d) borosilicate glass; (e) barium borate glass; (f) magnesium aluminum silicate; (g) ball clay; and (h) aluminum.
In accordance with yet another embodiment of the present invention, a resistor body for use in a resistor-type spark plug having a terminal stud and a center electrode, comprises (a) a resistor; (b) a first contact glass composition adjacent to the resistor and the terminal stud, the first contact glass composition comprising a mixture of (i) at least one electrically conductive material selected from the group consisting of nickel, copper, iron, zinc, titanium, silver, and oxides thereof; (ii) graphite; and (iii) silicon; and (c) a second contact glass composition adjacent to the resistor and the center electrode, the second contact glass composition comprising a mixture of (i) at least one electrically conductive material selected from the group consisting of nickel, copper, iron, zinc, titanium, silver, and oxides thereof; (ii) graphite; and (iii) silicon.
A more complete appreciation of the present invention and its scope can be obtained from understanding the accompanying drawing, which is briefly summarized below, the following detailed description of the invention, and the appended claims.